Vanadium haloperoxidases catalyze the two electron oxidation of a halide (Cl-, Br- or I-) by peroxide. For model systems, rate and mechanistic data for oxidation of Br- and I- have been reported, but not for Cl-. With both the enzymes and model systems, protonation of a peroxovanadate intermediate is essential for activity. The lack of reactivity for Cl- oxidation with model systems can be explained by considering the pK(a) of the protonated peroxovanadate complex. In water, a pK(a) near 0 prevents the active protonated complex from forming. In acetonitrile, the pK(a) of HCl(ca. 9) is greater than that of the protonated complex (ca. 5-6) and once again the active protonated complex is not formed. The primary goals of this proposal are to achieve chloride oxidation with model systems by manipulating the pK(a) of both the peroxovanadate complex and HCl through the use of mixed solvent systems (1-25% water in acetonitrile) so that the concentration of the active protonated species is maximized. At the optimum water concentration, the rate dependence on concentrations of V, H+, H2O2, and Cl- will be measured to obtain kinetic and mechanistic data for Cl- oxidation. This data will be compared with results reported for the enzymes.